Polyarylene sulfides (hereinafter, referred to as “PAS”) as represented by polyphenylene sulfides (hereinafter, referred to as “PPS”) are engineering plastics having excellent heat resistance, chemical resistance, flame retardant properties, mechanical strength, electrical properties, dimensional stability, and the like. PAS can be formed into various molded products, films, sheets, fibers, or the like by a general melt processing method such as extrusion molding, injection molding, compression molding, and the like. Therefore, the PAS is generally used in a wide variety of technical fields such as electrical apparatuses, electronic apparatuses, automotive apparatuses, packaging materials, and the like.
An alkali metal halide (for example, NaCl) produced as a byproduct when manufacturing PAS is disposed as wastewater. A sulfur source as an unreacted raw material is included in the byproduct. From the perspective of processing cost and environmental pollution prevention, the unreacted sulfur source is expected to be recovered and reused. For example, Patent Document 1 discloses a technique of washing a byproduct using a large amount of water, an acid is added to the washing liquid to produce hydrogen sulfide, the hydrogen sulfide is reacted with an alkali metal hydroxide, and then a recovered sulfidizing agent (sulfur source) is reused in a reaction.